Give me good reasons why R'lyeh is a balanced nation.

[Taqwus]

Returning (Thaug 1) and Ritual of Returning (Ench 5) will help save your Starspawn. In one ridiculously long SP game where by the end I was mostly romping around with Master Enslave, I had only one Starspawn priest summoner for the entire game -- he had over 60 summoning skill (GoH and Chalice were used to facilitate feeblemind recovery).
Greater Othernesses are definitely helpful; they're an unbreakable, very robust front line that doesn't care whether the opposition is mindless or not.
Re: archers, in addition to storms (which will not shut down archery, only hurt base PREC -- bonuses due to levels or magic are not affected IIRC), you could also use decoy units to throw off targeting, and perhaps try to find a Raptor province or other supply of fliers. If you can find a good air mage, Arrow Fend would also be a useful spell to cast.